Current mirrors are often used in analog circuits to replicate signal and bias currents. A current mirror may be characterized as having an output impedance which affects the accuracy of the current replicated in the current mirror. High output impedance in current mirrors is required for accurate replication of currents. In order to obtain a high output impedance, known current mirrors utilize cascode transistors. A current mirror may also be characterized as having an output voltage swing. High voltage swing in current mirrors is desired for accurate operation with low power supply voltages and for increased voltage signal amplitudes which improve the accuracy of analog circuitry utilizing the current mirrors.
FIG. 1 illustrates in schematic diagram form prior art high-swing cascode current mirror 10. High-swing cascode current mirror 10 includes current mirror 30 and cascode bias generator 20. Current mirror 30 includes N-channel cascode transistors 31 and 34 and N-channel transistors 32 and 33. N-channel transistor 31 has a source, a gate for receiving a bias voltage labeled "C.sub.NBIAS ", and a drain coupled to current source 23 for receiving an input current labeled "I.sub.IN ". N-channel transistor 32 has a source connected to a power supply voltage terminal labeled "V.sub.SS ", a gate connected to the drain of transistor 31, and a drain connected to the source of transistor 31. N-channel transistor 34 has a source, a gate for receiving bias voltage C.sub.NBIAS, and a drain for providing an output current labeled "I.sub.OUT ". N-channel transistor 33 has a source connected to V.sub.SS, a gate connected to the drain of N-channel transistor 31, and a drain connected to the source of transistor 34.
Cascade bias generator 20 includes N-channel transistor 21 and current source 22. Current source 22 has a first terminal connected to V.sub.DD, and a second terminal. Diode-connected N-channel transistor 21 has a source connected to V.sub.SS, a drain connected to the second terminal of current source 22 for providing bias voltage C.sub.NBIAS, and a gate connected to its drain.
Current mirror 30 is a conventional cascode current mirror which has high output impedance. The drain of cascode transistor 34 is the output terminal of prior art high-swing cascode current mirror 10. Cascade transistor 34 enhances the output impedance of the current mirror by reducing the voltage variation between the drain and source of transistor 33. Cascade transistor 31 maintains the same drain-source voltage drop across N-channel transistor 32 as that across N-channel transistor 33, which improves the current mirroring accuracy. Each of the transistors in prior art high-swing cascode current mirror 10 may be sized to minimize an excessive drain-source voltage drop across transistor 33 in order to provide a wide voltage swing at the output of the current mirror. For this reason prior art high-swing cascode current mirror 10 is useful in applications requiring either wide dynamic range or low voltage operation. The voltage swing at the output of prior art high-swing cascode current mirror 10 increases as bias voltage C.sub.NBIAS is reduced. However, bias voltage C.sub.NBIAS should be large enough to ensure that N-channel transistors 32 and 33 operate in saturation and that high output impedance is maintained. Therefore, the greatest output voltage swing that can be obtained while maintaining high output impedance occurs when bias voltage C.sub.NBIAS is equal to the sum of the gate voltage of N-channel transistor 33 and the minimum saturation voltage (V.sub.DSAT) of cascode N-channel transistor 34. The minimum saturation voltage is the minimum drain-source voltage that causes the transistor to operate in the saturation region.
Cascade bias generator 20 generates bias voltage C.sub.NBIAS using the gate-source voltage of diode-connected N-channel transistor 21. The current flow required in N-channel transistor 21 and current source 22 to generate bias voltage C.sub.NBIAS contributes a significant amount of the total power consumption of prior art high-swing cascode current mirror 10. In addition, cascode bias generator 20 requires additional layout area which makes the circuit more expensive to manufacture. Thus, a new high-swing current mirror is needed to decrease the power consumption, layout area, and manufacturing expense of analog circuits.